Some of the large marine transmissions that are presently in use have forward and reverse modes of operation, which is necessary because the gas turbine engines in use today are unidirectional power sources. In the past, steam engines, which were bi-directional, were more common.
Reversible marine transmissions generally include clutches in which interleaved plates are engaged under pressure. Prompt disengagement of the power source and modulation of the output speed are facilitated in this manner, these features being important when a ship is maneuvering. Most of the time, however, the transmission operates continuously in the ahead mode without employing these capabilities.
Modulated engagement clutches are sometimes problematic and are subject to being burned out, often when the ship is far from a suitable repair facility. When a clutch fails, a common practice is to effect a temporary repair by rigidly joining the input and output shafts of the clutch together, using an arrangement known as "get home bolts". This arrangement, however, presents the serious drawback that it precludes the availability of reverse. It also makes maneuvering difficult because of the absense of a disengaging clutch in the power train.
An objective of the present invention is to provide an improved marine transmission that employs modulated engagement clutches or brakes for maneuvering and reverse operations but avoids the consequences traditionally associated with the use of a modulated engagement clutch to transmit power during long periods of operation when the capabilities of such devices are not required. Another objective is to provide a marine transmission with redundancy in the forward mode of operation to improve its reliability. Still another advantage is to provide a marine transmission of relatively simple and reliable construction.